12,565 research outputs found
The two gap transitions in GeSn: effect of non-substitutional complex defects
The existence of non-substitutional -Sn defects in GeSn
was confirmed by emission channeling experiments [Decoster et al., Phys. Rev. B
81, 155204 (2010)], which established that although most Sn enters
substitutionally (-Sn) in the Ge lattice, a second significant fraction
corresponds to the Sn-vacancy defect complex in the split-vacancy configuration
( -Sn ), in agreement with our previous theoretical study [Ventura et
al., Phys. Rev. B 79, 155202 (2009)]. Here, we present our electronic structure
calculation for GeSn, including substitutional -Sn as
well as non-substitutional -Sn defects. To include the presence of
non-substitutional complex defects in the electronic structure calculation for
this multi-orbital alloy problem, we extended the approach for the purely
substitutional alloy by Jenkins and Dow [Jenkins and Dow, Phys. Rev. B 36, 7994
(1987)]. We employed an effective substitutional two-site cluster equivalent to
the real non-substitutional -Sn defect, which was determined by a
Green's functions calculation. We then calculated the electronic structure of
the effective alloy purely in terms of substitutional defects, embedding the
effective substitutional clusters in the lattice. Our results describe the two
transitions of the fundamental gap of GeSn as a function of the
total Sn-concentration: namely from an indirect to a direct gap, first, and the
metallization transition at higher . They also highlight the role of
-Sn in the reduction of the concentration range which corresponds to the
direct-gap phase of this alloy, of interest for optoelectronics applications.Comment: 11 pages, 9 Figure
Temperature and doping dependence of normal state spectral properties in a two-orbital model for ferropnictides
Using a second-order perturbative Green's functions approach we determined
the normal state single-particle spectral function
employing a minimal effective model for iron-based superconductors. The
microscopic model, used before to study magnetic fluctuations and
superconducting properties, includes the two effective tight-binding bands
proposed by S.Raghu et al. [Phys. Rev. B 77, 220503 (R) (2008)], and intra- and
inter-orbital local electronic correlations, related to the Fe-3d orbitals.
Here, we focus on the study of normal state electronic properties, in
particular the temperature and doping dependence of the total density of
states, , and of in different Brillouin zone
regions, and compare them to the existing angle resolved photoemission
spectroscopy (ARPES) and previous theoretical results in ferropnictides. We
obtain an asymmetric effect of electron and hole doping, quantitative agreement
with the experimental chemical potential shifts as a function of doping, as
well as spectral weight redistributions near the Fermi level as a function of
temperature consistent with the available experimental data. In addition, we
predict a non-trivial dependence of the total density of states with the
temperature, exhibiting clear renormalization effects by correlations.
Interestingly, investigating the origin of this predicted behaviour by
analyzing the evolution with temperature of the k-dependent self-energy
obtained in our approach, we could identify a number of specific Brillouin zone
points, none of them probed by ARPES experiments yet, where the largest
non-trivial effects of temperature on the renormalization are present.Comment: Manuscript accepted in Physics Letters A on Feb. 25, 201
Normal state electronic properties of LaOFBiS superconductors
A good description of the electronic structure of BiS-based
superconductors is essential to understand their phase diagram, normal state
and superconducting properties. To describe the first reports of normal state
electronic structure features from angle resolved photoemission spectroscopy
(ARPES) in LaOFBiS, we used a minimal microscopic model to
study their low energy properties. It includes the two effective tight-binding
bands proposed by Usui et al [Phys.Rev.B 86, 220501(R)(2012)], and we added
moderate intra- and inter-orbital electron correlations related to Bi-(,
) and S-(, ) orbitals. We calculated the electron Green's
functions using their equations of motion, which we decoupled in second-order
of perturbations on the correlations. We determined the normal state spectral
density function and total density of states for LaOFBiS,
focusing on the description of the k-dependence, effect of doping, and the
prediction of the temperature dependence of spectral properties. Including
moderate electron correlations, improves the description of the few
experimental ARPES and soft X-ray photoemission data available for
LaOFBiS. Our analytical approximation enabled us to
calculate the spectral density around the conduction band minimum at
, and to predict the temperature dependence of
the spectral properties at different BZ points, which might be verified by
temperature dependent ARPES.Comment: 9 figures. Manuscript accepted in Physica B: Condensed Matter on Jan.
25, 201
Dust from AGBs: relevant factors and modelling uncertainties
The dust formation process in the winds of Asymptotic Giant Branch stars is
discussed, based on full evolutionary models of stars with mass in the range
MMM, and metallicities .
Dust grains are assumed to form in an isotropically expanding wind, by growth
of pre--existing seed nuclei. Convection, for what concerns the treatment of
convective borders and the efficiency of the schematization adopted, turns out
to be the physical ingredient used to calculate the evolutionary sequences with
the highest impact on the results obtained. Low--mass stars with MM produce carbon type dust with also traces of silicon carbide. The
mass of solid carbon formed, fairly independently of metallicity, ranges from a
few M, for stars of initial mass M, to
M for MM; the size of dust
particles is in the range mm. On the contrary,
the production of silicon carbide (SiC) depends on metallicity. For the size of SiC grains varies in the range m, while the mass of SiC formed is
. Models of
higher mass experience Hot Bottom Burning, which prevents the formation of
carbon stars, and favours the formation of silicates and corundum. In this case
the results scale with metallicity, owing to the larger silicon and aluminium
contained in higher--Z models. At Z= we find that the most
massive stars produce dust masses M, whereas models of
smaller mass produce a dust mass ten times smaller. The main component of dust
are silicates, although corundum is also formed, in not negligible quantities
().Comment: Paper accepted for publication in Monthly Notices of the Royal
Astronomical Society Main Journal (2014 January 4
Non-substitutional single-atom defects in the Ge_(1-x)Sn_x alloy
Ge_(1-x)Sn_x alloys have proved difficult to form at large x, contrary to
what happens with other group IV semiconductor combinations. However, at low x
they are typical examples of well-behaved substitutional compounds, which is
desirable for harnessing the electronic properties of narrow band
semiconductors. In this paper, we propose the appearance of another kind of
single-site defect (), consisting of a single Sn atom in the center
of a Ge divacancy, that may account for these facts. Accordingly, we examine
the electronic and structural properties of these alloys by performing
extensive numerical ab-initio calculations around local defects. The results
show that the environment of the defect relaxes towards a cubic
octahedral configuration, facilitating the nucleation of metallic white tin and
its segregation, as found in amorphous samples. Using the information stemming
from these local defect calculations, we built a simple statistical model to
investigate at which concentration these defects can be formed in
thermal equilibrium. These results agree remarkably well with experimental
findings, concerning the critical concentration above which the homogeneous
alloys cannot be formed at room temperature. Our model also predicts the
observed fact that at lower temperature the critical concentration increases.
We also performed single site effective-field calculations of the electronic
structure, which further support our hypothesis.Comment: 12 pages, 1 table, 16 figure
On the alumina dust production in the winds of O-rich Asymptotic Giant Branch stars
The O-rich Asymptotic Giant Branch (AGB) stars experience strong mass loss
with efficient dust condensation and they are major sources of dust in the
interstellar medium. Alumina dust (AlO) is an important dust component
in O-rich circumstellar shells and it is expected to be fairly abundant in the
winds of the more massive and O-rich AGB stars. By coupling AGB stellar
nucleosynthesis and dust formation, we present a self-consistent exploration on
the AlO production in the winds of AGB stars with progenitor masses
between 3 and 7 M and metallicities in the range 0.0003 Z
0.018. We find that AlO particles form at radial distances from
the centre between and 4 R (depending on metallicity), which is in
agreement with recent interferometric observations of Galactic O-rich AGB
stars. The mass of AlO dust is found to scale almost linearly with
metallicity, with solar metallicity AGBs producing the highest amount (about
10 M) of alumina dust. The AlO grain size decreases
with decreasing metallicity (and initial stellar mass) and the maximum size of
the AlO grains is 0.075 for the solar metallicity models.
Interestingly, the strong depletion of gaseous Al observed in the
low-metallicity HBB AGB star HV 2576 seems to be consistent with the formation
of AlO dust as predicted by our models. We suggest that the content of
Al may be used as a mass (and evolutionary stage) indicator in AGB stars
experiencing HBB.Comment: 13 pages, 8 figures, accepted for publication in MNRA
AGB stars in the SMC: evolution and dust properties based on Spitzer observations
We study the population of asymptotic giant branch (AGB) stars in the Small
Magellanic Cloud (SMC) by means of full evolutionary models of stars of mass
1Msun < M < 8Msun, evolved through the thermally pulsing phase. The models also
account for dust production in the circumstellar envelope. We compare Spitzer
infrared colours with results from theoretical modelling. We show that ~75% of
the AGB population of the SMC is composed by scarcely obscured objects, mainly
stars of mass M < 2.5Msun at various metallicity, formed between 700 Myr and 5
Gyr ago; ~ 70% of these sources are oxygen--rich stars, while ~ 30% are
C-stars. The sample of the most obscured AGB stars, accounting for ~ 25% of the
total sample, is composed almost entirely by carbon stars. The distribution in
the colour-colour ([3.6]-[4.5], [5.8]-[8.0]) and colour-magnitude ([3.6]-[8.0],
[8.0]) diagrams of these C-rich objects, with a large infrared emission, traces
an obscuration sequence, according to the amount of carbonaceous dust in their
surroundings. The overall population of C-rich AGB stars descends from
1.5-2Msun stars of metallicity Z=0.004, formed between 700 Myr and 2 Gyr ago,
and from lower metallicity objects, of mass below 1.5Msun, 2-5 Gyr old. We also
identify obscured oxygen-rich stars (M ~ 4-6Msun) experiencing hot bottom
burning. The differences between the AGB populations of the SMC and LMC are
also commented.Comment: 18, pages, 11 figures, accepted for publication on MNRA
Characterization of active fault scarps from LiDAR data: a case 1 study from Central 2 Apennines (Italy)
A high resolution DEM (1 ms spacing) derived from an airborne LiDAR campaign was
11 used in an attempt to characterize the structural and erosive elements of the geometry of the Pettino 12 fault, a seismogenic normal fault in Central Apennines (Italy). Four 90- to 280 m -long fault scarp 13 segments were selected and the surface between the base and the top of the scarps was analyzed 14 through the statistical analysis of the following DEM-derived parameters: altitude, height of the 15 fault scarp, distance along strike, slope and aspect. The results identify slopes of up to 40° in faults 16 lower reaches interpreted as fresh faces, 34° up the faces. The Pettino fault maximum long slipe17 rate (0.6-1.1 mm/yr) was estimated from the scarp heights, which are up to 12 and 19 m in the 18 selected four segments, and the age (ca. 18 ka) of the last glacial erosional phase in the area. The 19 combined analysis of the DEM-derived parameters allow us to (a) define aspects of 3D scarp 20 geometry, (b) decipher its geomorphological significance, and (c) estimate the long-term slip rate
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